This application is based on and claims priority of German patent application No. 19952373.8 filed on Oct. 30, 1999 and German patent application No. 10037380.1 filed on Aug. 1, 2000, the disclosures of each of which are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to an apparatus and a method for measuring the concentration of a paramagnetic gas in a gas sample, especially oxygen in respiratory gas.
2. Description of the Related Art
From the publication by H. Torwegge entitled xe2x80x9cDie Einwirkung magnetischer Felder auf das Wxc3xa4rmeleitvermxc3x6gen von NO und NO2xe2x80x9d [xe2x80x9cThe Influence of Magnetic Fields on the Thermal Conduction Capacity of NO and NO2xe2x80x9d], Annalen der Physik [xe2x80x9cAnnals of Physicsxe2x80x9d], 5th series, Vol. 33, 1938, pages 459-470, it is known that paramagnetic gases vary their thermal conductivity under the influence of magnetic fields. The reason for this behavior is evidently the fact that paramagnetic gases possess a permanent magnetic moment, which however normally, because of the thermal molecular motion of the gas molecules, is not apparent from outside. These conditions change if a sufficiently strong, external magnetic field assures that the magnetic moments of the individual molecules are oriented. On the one hand, this causes a change in susceptibility, which leads to an increase in the magnetic flux; on the other hand, a certain molecular order ensues in the gas, thereby limiting the capability of transmitting heat energy to adjacent molecules by impacts. This changes the thermal conductivity slightly.
In the known apparatus, the gas sample to be examined is located in a cylindrical vessel, in whose longitudinal axis a thin measuring wire, heated to an operating temperature, is disposed. If the thermal conductivity of the gas varies because of an external magnetic field, this causes a change in resistance of the measuring wire, which can be determined with a measurement bridge. A disadvantage of the known apparatus is that because of the requisite precision in detecting temperature changes of the measuring wire, very stringent demands must be made of the resistance measurement. This makes it impossible to distinguish the actual measurement signal unequivocally from the noise variables and drift factors that are always present.
It is the object of the invention to improve an apparatus of the type defined above in such a way that the noise variables that impair the measurement signal are largely eliminated, and to disclose a method for performing the measurement.
The object with regard to the apparatus is attained with the characteristics of claim 1. This object is also attained with the characteristics of claims 16, 19, 21, and 22.
The object with regard to the method is attained with the characteristics of claim 14.
In the measuring apparatus of the invention, the measuring element, heated to the operating temperature, is disposed in a modulated magnetic field, and the periodic fluctuations of the heat flow measurement signal that occurs at the measuring element and that are based on the modulation of the magnetic field source are evaluated. The periodic fluctuations of the heat flow measurement signal are especially simple to distinguish from the background signal by means of a filter device. By selective evaluation of the periodic fluctuations, noise factors are largely eliminated, since the effects of drift and temperature are contained predominantly in the direct component of the heat flow measurement signal.
The heat flow to be evaluated, that is, the quantity of heat dissipated from the measuring element to the gas sample, is composed of the thermal conduction and the thermal capacity of the gas sample together and is dependent on the proportion of the paramagnetic gas.
The magnetic field can be modulated mechanically, for instance with rotating permanent magnets, or electrically. For the electrical modulation, alternating voltages that are low in harmonics, such as sine wave voltages, are especially suitable.
It is especially advantageous, instead of a single magnetic field source, to use two modulatable magnetic field sources and to connect them in alternation to the modulation source, so that at all times, one magnetic field source is free of current and the other is acted upon by the modulation current. The measuring element is disposed inside the air gaps of the magnetic field sources and for the case of a thermocouple arrangement has opposed first and second connection points. Depending on the current imposed on the magnetic field sources, the connection points, which are heated to the same working temperature, which is elevated compared to the temperature of the gas sample, act as a measuring and compensation element. In the presence of a paramagnetic gas in the measuring chamber formed by the air gaps, the thermal conduction in the air gap of the magnetic field source on which current is imposed decreases, resulting in a corresponding temperature increase and leading to a differential temperature between the connection points. This differential temperature can be measured as the thermocouple voltage. It is especially advantageous that fluctuations in thermal conduction of the gas sample stream have the same effect on both connection points and are thus compensated for.
Advantageous features of the invention are defined by the dependent claims.
For measuring the heat flow from the measuring element to the gas sample, an arrangement of one or more thermocouples, which are placed in the air gap inside the magnetic field, has proved especially advantageous. In a thermocouple arrangement with three connection points, one of the connection points is for instance located in the center of the wire, while the other two connection points are disposed at the support wires leading to the thermocouple arrangement. If the thermocouple arrangement is now heated by an alternating current source to an operating temperature elevated compared to the temperature of the gas sample, then this heats the support wires only insignificantly, because they are dimensioned substantially thicker than the thermocouple wire. Thus the connection points placed at the support wires are approximately at the ambient temperature level. The thermocouple voltage, as the intrinsic electromotive force (EMF) of the thermocouple arrangement, is thus proportional to the operating temperature of the thermocouple wire. Since the thermocouple voltage remains substantially uninfluenced by the intensity of the magnetic field, the temperature measurement is not interfered by the magnetic field modulation.
It is especially advantageous to utilize the thermocouple voltage to regulate the alternating current source. If the temperature difference between the thermocouple arrangement and the gas sample to be examined is regulated to a constant value, then the electrical power required for this corresponds to the quantity of heat dissipated by thermal conduction into the gas sample and via the support wires.
As an alternative to regulating to a constant temperature difference between the thermocouple arrangement and the gas sample, the electrical power, converted in the thermocouple arrangement, or the heating alternating current, heating alternating voltage, or Ohmic resistance of the thermocouple arrangement can be kept constant, and the thermocouple voltage that ensues can be further processed as the heat flow measurement signal.
Besides the electrical modulation of the magnetic field source, the possibility exists of varying the magnetic field with a rotating chopper disk of soft magnetic or permanent magnetic material disposed inside the air gap.
An advantageous method for measuring the proportion of a paramagnetic gas in a gas sample comprises disposing a measuring element, heated to an operating temperature, in an air gap, receiving the gas sample, of a modulated magnetic field source; determining a heat flow measurement signal from the heat flow from the measuring element to the gas sample; with a filter device, filtering out periodic fluctuations from the heat flow measurement signal that are caused by the modulation of the magnetic field source; and from the periodic fluctuations, determining a concentration measurement value that indicates the proportion of the gas.
Especially advantageously, with the method disclosed according to the invention, oxygen concentrations in medical therapeutic appliances can be determined, since in contrast to the electrochemical sensors usually used in such equipment, the measuring element is not subject to any wear.
One exemplary embodiment of the invention is shown in the drawing and described in further detail hereinafter.